The blood flukes Schistosoma mansoni and Schistosoma haematobium are major causes of hepatic and urinary tract disease in Africa and the Middle East. To aid in development of prevention and control measures for schistosomiasis (bilharzia), a serologic test of human exposure to the infectious larvae of the parasite will be developed based on a protease antigen released at the initial stage of human infection. cDNAs coding for both the S. mansoni and S. haematobium protease antigens were expressed in E. coli or yeast to provide a standardized source of antigen protein for an ELISA-based test. The antigens from each individual species, as well as a combination of the two, will be evaluated to optimize detection. Once the optimal antigen preparation is identified, a longitudinal field study will be carried out to evaluate the efficacy of the ELISA-based assay. This work is a collaboration between laboratories at the University of California, San Francisco, and Ain Shams University in Cairo, Egypt. The field trials are being carried out in the Nile Delta region where both infections are highly endemic. Development of this assay will provide a necessary tool to identify foci of drug resistance, evaluate therapeutic efficacy, identify new outbreaks of disease, and evaluate potential antischistosomal vaccines.